In Mail, just right-click (Control+click) a link in an email and select
'Quick Look URL' from the contextual menu.

[crarko adds: When I tried this the Data Detectors in Mail identified some URLs and gave me this option. Curiously, with others right-clicking on them just opened the link in Safari. I'm not sure how it made the distinction. Also, the time it took for the Quick Look preview to appear was pretty long. It's probably just as effective to open the link in a browser. Nevertheless, it is a feature to document, so here it is.]

Just hovering over the link in Mail will give you a Data Detector selection arrow. Clicking that automatically opens a Quick Look view of the web page, with an "Open in Safari" option. The pages I've viewed so far have opened quite quickly in Quick Look.

In my experience, hovering over a link causes a little down arrow icon to appear at the right end of the link. Clicking on this brings up a QuickLook preview of the page. Clicking on the preview takes you to the page in Safari. No right-clicking necessary.

The advantage to this as apposed to just clicking the link and going to Safari is that you are not abruptly switching contexts. You're staying in Mail. If you want to then open the page but stay in Mail, you can select "Open Link Behind Mail" from the contextual menu and continue with your email reading.